Constraints on Aerosol Nitrate Photolysis as a Potential Source of HONO and NOx

The concentration of nitrogen oxides (NOx) plays a central role in controlling air quality. On a global scale, the primary sink of NOxis oxidation to form HNO3. Gas-phase HNO3photolyses slowly with a lifetime in the troposphere of 10 days or more. However, several recent studies examining HONO chemistry have proposed that particle-phase HNO3undergoes photolysis 10–300 times more rapidly than gas-phase HNO3. We present here constraints on the rate of particle-phase HNO3photolysis based on observations of NOxand HNO3collected over the Yellow Sea during the KORUS-AQ study in summer 2016. The fastest proposed photolysis rates are inconsistent with the observed NOxto HNO3ratios. Negligible to moderate enhancements of the HNO3photolysis rate in particles, 1–30 times faster than in the gas phase, are most consistent with the observations. Small or moderate enhancement of particle-phase HNO3photolysis would not significantly affect the HNO3budget but could help explain observations of HONO and NOxin highly aged air.